300 



CEPHALOPODA. 





The lens of the Cephalopodan eye is not yielded by the outer wall 

 of the primitive optic vesicle alone, since the floor of the second 

 invagination above the vesicle also takes part in its formation (Fig. 

 143). From this latter is formed an anterior and smaller section of 

 the lens (Fig. 144 A and B). The cell-layers formerly lying above 



the first-formed lens are in 

 this way gradually used up 

 (Figs. 143 and 144). 



The primitive optic 

 vesicle and the invagination 

 lying above it also }deld the 

 ciliary body, the wall of the 

 former, which is directed 

 outward, and the floor of 

 the latter uniting for its 

 formation (Fig. 144 .4 and 

 B). The mesoderm present 

 between these two ecto- 

 dermal cell-layers no doubt 

 a;ives rise to the musculature 

 of the ciliary body. The 

 anterior or outer part of 

 the invagination (or rather 

 fold) becomes the iris (Figs. 

 144 and 145). Mesoderm- 

 elements find their way in 

 large numbers into this 

 anterior fold also. 



Another circular fold now 

 grows over the eye at this 

 stage of its development 

 and gives rise to the cornea 

 (Fig. 145 C). This fold, in 

 many Cephalopoda {e.g., 

 Oigopsida) does not close, 

 the cornea retaining its 

 aperture which is often somewhat wide and through which the 

 sea water can enter the external optic chamber, while in others 

 {e.g., the Myopsida among the Decapoda and the Octopoda), the 

 cornea either retains only a minute aperture or else completely closes, 

 thus precluding any communication between the optic chamber and 



Flu. 144. -Sections through the eye of Loligo 

 in two stages of development (after BOBRKTZKY, 

 from Balfour's Text-book). <* and "', the 

 epithelium lining the anterior optic chamber ; 

 ni. and if, iris fold : aq, equatorial cartilage ; 

 cc, small ectoderm-cells of the ciliary body , 

 </;, large cells of the ciliary body (///) ; /'/, 

 inner part of the lens ; ms, mesoderm-tissue of 

 the ciliary body ; rt, inner, rt", outer layer of 

 the retina; st, rods; vl ', anterior part of the 

 lens; x, epithelium of funnel. 



